Commercial vat dyeing is usually carried out in an aqueous alkaline bath in which the reduced form of the dye, the so-called leuco form, is dissolved. Fibers in the form of fabric, or warp yarns, are passed through the bath with a residence time such as to permit the fibers to absorb the desired amount of leuco dye. The fibers are then removed from the bath and exposed to air which oxidizes the dye to the insoluble form. Because leuco indigo, unlike most other vat dyes, has only limited affinity for cellulose, dyeing is especially difficult. In order to obtain adequate depth of shade, corresponding, for example, to 1.6 wt. % fixed indigo, it is trade practice to pass the fibers through a train of four to six dip vats containing bath liquor comprising leuco indigo, sodium hydrosulfite reductant and sodium hydroxide. Following each dip, the fibers are squeezed between rolls, thus reducing the bath liquor pickup to about 65% on the weight of the fiber. Thereafter, the fibers are carried over rolls in the open air, called a greening passage, a process also known as "skying," where the leuco indigo is oxidized to the insoluble form thus fixing it in the fibers. The residence time of the fibers in and the temperature of the dip vats are selected to allow soluble leuco indigo to penetrate the fibers yet not reconvert substantial amounts of indigo already fixed to the soluble leuco form. Typically ambient temperature and temperatures as high as 35.degree. C. are employed. A dip bath residence time of 60 seconds is typical. On leaving the dyeing train, the fibers are scoured and dried.
Oxidation in the dip baths of the vat dye liquor, which, of course, is undesirable, is troublesome in all vat dyeing but is particularly so in indigo dyeing. Oxidation occurs where the liquor contacts the air for example at the horizontal air-liquor interface, and especially in the region of the squeeze rolls where multiple interfaces are generated exposing large areas of liquor to the air through draining of expressed liquor and splashing.
This unwanted oxidation results in significant loss of expensive sodium hydrosulfite and sodium hydroxide and the formation of troublesome floating scum of reoxidized insoluable dye which often cause spotting and uneven dyeing.
It has long been recognized that substitution of an inert gas for oxygen-containing air contacting the indigo dyebath would avoid the above problem. Already in 1838 Woodcroft suggested in U.S. Pat. No. 678 the use of air from which the oxygen has been depleted by contacting with sulfuret of lime. It was suggested to print cotton fabric with leuco indigo in a chamber containing air from which the oxygen has been removed in the above manner. An operator inside the chamber was to be provided with an enclosing impervious suit fitted with hoses for providing breathing air from outside the chamber. The system has not been adopted in the trade.